1. Field of the Invention
This invention relates to a device for accurately installing the surface of a planar object such as a semiconductor wafer perpendicularly to the optic axis of an object lens, and in particular to a device for detecting the levelling of the surface of an object.
2. Related Background Art
In the lithography process in the manufacture of semiconductive integrated circuits, use is made chiefly of a reduction projection type exposure apparatus using the step-and-repeat system, i.e., a so-called stepper, and circuit patterns formed on a mask or a reticle (hereinafter referred to as the reticle) are successively exposed onto a wafer which is a piece to be worked through a projection objective lens (hereinafter simply referred to as a projection lens). A projection lens having a great numerical aperture (N.A.) is generally used in this stepper and therefore, the allowed focus range is very small. So, in order to accomplish the exposure of clear-cut patterns over the entire exposure area on the wafer, the center or a marginal point of the exposure area is usually detected for each exposure area by the use of an auto focus mechanism, and the exposure area is moved in the direction of the optic axis of the projection lens so as to be perpendicular to the optic axis of the projection lens. Recently, however, the numerical aperture (N.A.) of the projection lens is further increased correspondingly to the minimum line width of a circuit formed in the order of sub-micron, and an inclination relative to the imaging plane of the projection lens occurs in the other area than the detected point on the exposure area by the auto focus mechanism, and the occurrence of partial focus deviation in the exposure area caused by this inclination poses a problem. Therefore, it is necessary to maintain the surface of the exposure area perpendicular to the optic axis of the projection lens. As a conventional device for detecting the horizontal, there is a collimator type "horizontal position detecting device" as disclosed, for example, in U.S. Pat. No. 4,558,949. This horizontal position detecting device includes an irradiating optical system for supplying a parallel light flux emitted from a minute opening to an area of the surface of an object from a direction oblique relative to the optic axis of a main objective lens, and a condensing optical system for condensing on a light receiving element the light flux supplied from said irradiating optical system and reflected by said area, the optic axes of said two optical systems being disposed symmetrically with respect to the optic axis of said main objective lens, and whether said area is perpendicular to the optic axis of the main objective lens is discriminated from the output of the light receiving element, whereby the levelling of the surface of the object is detected.
In such a levelling detecting device, it is often the case that the shape of the detection area on the surface of the object is fixed and that shape is generally a circular area inscribed or circumscribed with the area.
The conventional projection exposure apparatus carrying thereon a levelling detecting device in which the detection area is fixed as described above has suffered from the following problem. When the outer edge portion of a wafer is exposed, a part of the exposure area sometimes protrudes from the wafer. At this time, the levelling detecting device detects the horizontal of a partly vacant area. However, such outer edge portion of the wafer often has its surface inclined. Particularly, where pattern lithography having a plurality of chip patterns in one projection exposure area is carried out, when there is a chip pattern exposed in a perfect form on the wafer without protruding from the wafer even in the case of exposure in which the part as described above protrudes from the wafer, it is required in manufacturing efficiency to make the most of that chip pattern. In such a case, the inclination of the surface of the outer edge portion of the wafer as described above adversely affects the detection of the horizontal.